The pink is biology, not mineral. Lake MacDonnell is a salt lake, and in its dense brine lives a microscopic alga called Dunaliella salina, along with salt-loving microbes known as halobacteria. Under intense sun and extreme salinity, these organisms produce huge quantities of beta-carotene - the same red-orange pigment that colours carrots - as a kind of biochemical sunscreen against the harsh light. Multiply that pigment across an entire shallow lake and the result is the saturated rose hue that draws photographers from around the world. The brilliance comes and goes: the colour is strongest when the water is shallow and the salt most concentrated, and fades as levels rise. Beside the pink, a green lake and, further along, a blue one complete an unlikely natural palette laid out within a few hundred metres of road.

What makes Lake MacDonnell spectacular from above makes it valuable from below. The lakebed sits atop the largest gypsum deposit in the southern hemisphere - an immense body of calcium sulphate that may hold somewhere between 500 and 700 million tonnes spread across roughly 87 square kilometres. A surface layer of gypsarenite some 93 percent pure gives way to a thick band of selenite running 94 to 96 percent calcium sulphate beneath. The gypsum formed in geologically recent times, during the Holocene, atop older Pleistocene coastal dunes - a reminder that this flat, sun-struck country was shaped by an ancient, shifting shoreline.

Gypsum has been worked here since 1919, and the operation is no small affair: around 3.5 million tonnes are extracted each year, making this the largest gypsum mine in Australia. Salt, once the main product, is now mined only as a secondary one. The raw gypsum is stockpiled at nearby Thevenard, then shipped east to Glebe Island in Sydney for processing into plaster and wallboard. The local railway ends at a point with the curiously human name of Kevin - drawn from the Hundred of Kevin, itself named after the adopted son of a 19th-century South Australian politician. The terminus was little more than a small corrugated-steel waiting shed, and so few people lived nearby that it was almost never used. Today the mine works on, quiet and industrial, beside a lake that millions know only as a splash of impossible pink.

The lake's fame rests on a single, photogenic accident of geography: the causeway. A narrow road runs straight across the water, and where it does, the pink lake on one side meets the green lake on the other in a hard, startling line - rose and emerald divided by a strip of gravel barely wide enough for a car. It is the shot that has carried Lake MacDonnell around the world, a place where you can stand with one colour at each shoulder. Reaching it means turning off the Eyre Highway near Penong and following the road toward the coast at Point Sinclair, out where the western Eyre Peninsula runs down to the sea. The colour, though, keeps its own counsel. It blazes brightest when the lake is shallow and the salt concentrated under a high sun, and dims when the water deepens - so the spectacle is never quite guaranteed, which is part of what makes catching it feel like a small piece of luck.